1. Field of the Invention
The present invention relates to a diffractive optical element (“DOE”), an optical system, and an optical apparatus.
2. Description of the Related Art
One known layered DOE has a structure in which a plurality of diffractive gratings is arranged, and a grating height and a material of each diffraction grating are properly set (Japanese Patent No. 3,717,555). It is also known to use a material having a linear abnormal dispersion property or a value of a partial dispersion ratio θgF smaller than that of a usual material for a diffractive efficiency of 99% or higher in an overall visible wavelength range (Japanese Patent Laid-Open Nos. (“JPs”) 2004-78166 and 2008-241734).
JPs 2004-78166 and 2008-241734 use a material in which ITO (Indium-Tin Oxide) nanoparticles are dispersed in resin, for a material having the linear abnormal dispersion property. The refractive index of ITO is changed by the electron transition and free carrier caused by tin doping and oxygen holes. A strong linear dispersion property reveals due to the electron transition and the free carrier. ITO is used for a transparent electrode, and known as a material having a relatively high transmittance. However, ITO is not sufficient for an optical system that is required to have a higher transmittance. A drop of the transmittance of ITO is caused by tin doping and, it is extremely difficult to obtain a material having a strong linear dispersion property and an extremely high transmittance. As a consequence, a transmittance difference occurs in a single grating between a part having a high absolute value of a grating height of the ITO dispersed material and a part having a low absolute value of the grating height. The transmittance difference in the single grating in the DOE is not problematic when a light flux is wider than an interval (pitch) between the gratings, but the luminance becomes uneven on an image plane as the light flux becomes narrow. The diffractive efficiency deteriorates when a mixture ratio of ITO nanoparticles decreases.